Fix 2005/mikeash Makefile CSILENCE

The option -Wno-extra-semi is reported in linux to be 'valid for
C++/ObjC++ but not for C'.

2005/mikeash/Makefile

@@ -38,7 +38,7 @@ include ../../var.mk
 
 # Common C compiler warnings to silence
 #
-CSILENCE= -Wno-error -Wno-implicit-function-declaration -Wno-extra-semi \
+CSILENCE= -Wno-error -Wno-implicit-function-declaration \
 	  -Wno-unknown-escape-sequence -Wno-unused-variable -Wno-pedantic \
 	  -Wno-builtin-declaration-mismatch -Wno-misleading-indentation \
 	  -Wno-unknown-warning-option